In continuous ink-jet printers,each character of print is made up of plural lines of droplets which extend in a direction transverse to the direction of relative movement between the printer and the substrate. Each line is printed from a so-called `raster` of droplets in which each printable droplet has a defined print position. Non-printable `guard` droplets separate the printable droplets and the printable droplets are either printed or not, depending on the character being formed.
Whilst such printers are capable of printing at very high speeds (droplets may be generated at 64kHz or even 128kHz), a particular problem often exists with the quality of printing produced by such printers, particularly where they are being used to print, from a single nozzle, a wide variety of very different characters or different character fonts. The characters cannot readily be printed to `near letter quality` (NLQ), unlike so-called `drop-on-demand` (DOD) ink-jet printers which use an array of closely spaced nozzles, with ink droplets from plural nozzles being used to make up each line of each character. Often, the lines of characters are slightly sloped, which itself may be undesirable, but the more significant problem is simply that the droplet resolution is not sufficiently high to allow characters to be well and fully shaped.
There has long been a desire to overcome this problem and techniques involving the use of multiple CIJ printheads have been proposed as one solution to the problem. Whilst reasonable resolution may be achieved, the increased cost and complexity of such printers rarely makes their use viable, particularly in marking and coding applications.
U.S. Pat. No. 4,670,761 discloses a continuous ink-jet character printing method which comprises providing a stream of droplets from a nozzle of a continuous ink-jet printhead; moving a print substrate past the printhead, said printhead having a pair of deflection electrodes for deflecting individual charged droplets from said stream of droplets to a required print position on the substrate; determining the speed of movement of the print substrate relative to the printhead; said deflection electrodes being disposed at a preselected angle relative to the path of movement of said print substrate, wherein the droplets are charged in dependence upon the speed of the substrate relative to the printhead to determine their print position.